1. Field of the Invention
This invention relates to the fabrication of hard disk drives (HDD), particularly to a method of measuring head protrusion in a slider produced by the controlled use of heater elements.
2. Description of the Related Art
As magnetic read/write heads have been required to deal with magnetic media having increasingly higher area density of recorded information, various methods have been developed to improve the capabilities of the head to read and write at those levels. Traditionally, the direction taken in trying to achieve the reading and writing of this high density information has been to decrease the spacing (i.e. the static fly height) between the disk and the slider in each new generation of products.
FIG. 1 is a schematic illustration showing a single suspension-mounted slider (the combination collectively termed a “head gimbals assembly (HGA)”) positioned above a rotating magnetic hard disk during disk-drive operation in a hard disk drive (HDD) at ambient operating temperature. The suspension (101) holds the slider (10) at an angle above the surface of the spindle-mounted magnetic disk (400), producing a static “fly height” between the air bearing surface (ABS) (100) of the slider and the disk. A read/write head (600) is mounted within the slider. The rotation of the disk (400) is, by definition, into the leading edge of the slider, while the read/write head (600) is located at the trailing edge of the slider. The write-gap (from which the write magnetic field contacts the disk) of the head (90) is “above” (i.e. to the trailing edge side of) the read-gap portion (30). The hydrodynamics of the air layer between the ABS and the rotating disk surface supports the slider at a static fly height above the disk. In the two-heater dynamic flying height (DFH) type of system to be discussed herein, separately controllable heater elements (35), (95) are located adjacent to the two gaps (30) and (90) and, by heating the region surrounding the gaps, can cause protrusions (not shown) of the ABS (200) of the head portion relative to the undisturbed shape of the ABS. These protrusions will produce a characteristic shape (the protrusion profile) across the ABS, which will manifest itself in a corresponding profile of the flying height of the ABS above the disk (the flying height profile).
Although FIG. 1 illustrates one dual-heater DFH head, single-heater heads are common and multi-heater heads with more than two heaters can also be envisioned. It is further noted that hard disk drives (HDD) in the current art will typically include a plurality of adjacent, independently operating suspension mounted sliders exactly as shown in FIG. 1, each located adjacent to its own disk, which are mounted in a stack configuration. Although such an arrangement is not illustrated herein, it is easily visualized.
The limit of the total clearance budget (i.e. the sum of all factors entering into the total clearance between the head and the disk) prohibits a continuous reduction of this static fly height beyond a certain point. In addition to the static fly height variations resulting from the ABS and HGA/HSA (head gimbal assembly/head stack assembly) manufacturing processes, other factors also contribute to the total clearance between the head and the disk. A simple example is the drop in static fly height when the HDD is moved from sea level to a higher altitude. Yet another example is the isothermal PTP (pole tip protrusion) associated with the change in ambient temperature of the environment in which the HDD is located. Furthermore, the writer coil induced PTP (protrusion caused by joule heating of the coil) also diminishes the fly height clearance when the coil is activated to produce magnetic flux in a HDD write operation. There is a clear necessity to have a method of producing DFH (dynamic fly height) control, i.e. a method of providing a controllable head-disk spacing under various operational conditions, to avoid incidental contacts between the head and the disk that result from these inevitable variations in static fly height.
A common prior art approach to introducing such a “dynamic” control of fly height spacing is to embed a thin layer of heater film inside the magnetic recording head to produce single or multiple heater elements. The heater film is electrically connected to the pre-amplifier within which a heater current is activated to increase the heater film temperature and, thereby, to increase the temperature of the surrounding materials of the head structure. When subjected to this increased temperature, the materials forming the head begin to expand in accordance with their respective thermal expansion characteristics. This leads to a thermally deformable ABS and a resulting protrusion profile that achieves a lower spacing (less clearance) between the disk surface and the RG (read gap) and WG (write gap), thus greatly improving head performance.
When the read/write operation is not required, the heater current is turned off so that the ABS is elastically returned to its original, non-deformed state. The induced rise in temperature produced by the heating is sufficiently mild that the reliability of the head is not detrimentally affected. In addition, the heater activation has not shown a degrading effect on the magnetic reader in terms of noise and stability, since the magnetic fields produced by the heater activation currents is minimal.
The utilization of the DFH heater (or heaters) shows an unequivocal improvement in HDD performance. However, the same DFH power setting cannot be expected to deliver the same changes in spacing for each individual head due to the inevitable variations in the manufacturing process.
Single heater induced head element protrusion height is proportional to heater power and there is a fixed protrusion profile shape. Referring to FIG. 2, there is shown, schematically, a protrusion profile produced by a single heater as a function of distance along the ABS measured in microns from the edge (at x=0) between the alumina of the head element and the slider substrate edge (see (600) in FIG. 1). Different heater powers are shown, ranging from a low of 20 mW (milliwatts) (20) to a high of 100 mW (100). Each profile has a similar shape, displaying a flat region between the location of the read-gap and the write-gap and gradual increase and decrease in the protrusion to either side of the flat region. Typically the head element also includes a head-disk interference (HDI) sensor. This sensor is a resistive temperature sensor used to detect a temperature change in the head that is induced by changes in clearance during head vibrations caused by contact with disk asperities. In the single heater DFH, the head element flying profile is fixed between DFH touch-down and the flying operation.
Referring to FIG. 3, there is shown, schematically, a flying height profile graph with a single heater DFH, in which the height of the head above the medium (the ordinate) is measured in nanometers (nm) and length along the ABS of the head (abscissa) is measured in microns, exactly as in FIG. 2. The upper graph (5), representing a condition of no DHF (no heater activation), shows the passive flying height of the read gap and write gap above the medium surface. The lower graph, broken line (7), shows the effects of activating DFH during operation, which, in the case of a single heater, produces a fixed protrusion shape whose height above the medium is proportional to the power supplied to the single heater. Since the head touch-down point and the operational minimal fly height point are the same and cannot be adjusted, DFH touch-down detection and operational read-gap and write-gap clearances cannot be optimized. The solid line (9) beneath the broken line (7) shows the head touch-down used to calibrate the touch-down DFH power and to determine the operational DFH power. Due to the effects of head manufacturing tolerances on passive flying height and pole tip recession (PTR), head element DFH touch-down locations will be tolerated. This affects the head touch-down vibration and DFH touch-down detection by HDD or HDI sensor. Moreover, read gap and write gap clearances associated with variations in head element TD locations are also tolerated.
Referring to FIG. 4, there is shown a histogram of flying height (FH) clearance differences (“deltas”), defined as read-gap fly height (RG/FH) minus write-gap fly height (WG/FH), (delta=RG/FH−WG/FH) between read-gaps and write-gaps and the medium surface during touch-down and head flying operation. Not unexpectedly, the histogram is well represented by a Gaussian curve and shows that the greatest number of read/write heads show a clearance difference of 1.0 nm.
This distribution of clearance differences in single heater DFH that results from normal manufacturing tolerances directly affects the magnetic performance or recording density due to the variation in magnetic spacing between the reader and the disk. Read-gap and write-gap operation clearance sigma (standard deviations) would be increased by the touch-down detection error and head manufacturing tolerances (eg. PTR tolerances). Moreover, the distribution also exacerbates head vibration inconsistencies during DFH touch-down, causes HDI sensor touch-down detection error and ultimately affects head reliability.
To improve the operation clearance sigma and the HDI sensor touch-down detection accuracy, an adjustable DFH protrusion shape can be applied to the head element through multi-heater fly height (DFH) control or other features. Although such adjustable DFH protrusion shape is available, there is no methodology to calibrate suitable DFH protrusion shapes in order to obtain data necessary for establishing read and write gap clearances, to optimize the write-gap clearance, during reading, for 1st sector writing readiness, and to have a consistent level of DFH touch-down vibration.
To achieve the required improvement of DFH head performance, an effective calibration methodology of a variable DFH protrusion is a necessity. Although the following prior arts have considered aspects of these problems, they have not considered aspects addressed by the present invention nor have they arrived at the solution method of the present invention.
U.S. Pat. No. 7,492,543 (Mitsunaga et al) discloses a method to measure heater control values for each head. A clearance control unit varies electric power distribution to a heater to vary the protrusion value of the head to control the clearance.
U.S. Pat. No. 7,903,365 (Watanabe) teaches that clearances between the ABS surfaces and magnetic heads can be adjusted by respective heaters. A vibration detection sensor is disclosed.
U.S. Pat. No. 7,817,372 (Takahashi) shows a flying height control method including measuring saturation characteristics and adjusting flying height based on the measurements.
U.S. Pat. No. 7,468,854 (Yamashita et al) describes accurate measurement of flying height by controlling power distribution to the heaters in the heads.